1970s Hong Kong was flooded with heroin. He made a landmark film about it

From a darkened first-floor shop window, the zoom lens picks up the lean, sallow-faced figure loitering on the opposite pavement. Seemingly invisible to passers-by and shopkeepers, he is a magnet for solitary, middle-aged men who approach in silence, holding HK$10 notes.

No words are exchanged, but between meetings, he disappears into an open staircase and returns within moments to discreetly hand off small paper packets to more customers who then drift away. This is repeated, again and again, on a daily basis, with the hidden camera capturing every one of the transactions.

The team behind the camera are award-winning British filmmaker Adrian Cowell, two-time Oscar-winning cinematographer Chris Mengis, and a young local, Peter Yung Wai-chuen, associate producer and camera operator. They would live in that empty shop for five months, filming the street-level drug trade below.

“I was the organiser for the documentary,” says Yung, now 75, sitting among the greenery of his home on Lantau Island. “The important thing is, at that time, Caucasians couldn’t go on the surveillance, so that’s why I came in and became the one to deal with all these Chinese in the gangs.”